In optical communication systems, an optical signal is often supplied to a receiver including diode, such as a photodiode, which converts the optical signal into a corresponding electrical current. The current, in turn, may be supplied to a resistor to supply a corresponding output voltage for further processing by known circuitry.
The photodiode may have relatively high capacitance, as well as impedance. Accordingly, if the resistor has a relatively high resistance, a relatively high voltage can be generated which has a high signal-to-noise ratio. The response of the output voltage, however, can be slow due to coupling with the photodiode capacitance. Alternatively, if the resistor has a relatively small resistance, the magnitude of the output voltage will be reduced and may have a low signal-to-noise ratio.
Thus, conventional optical receivers may often incorporate an operational amplifier (or “opamp”) configured as a transimpedance amplifier. The opamp has an input that is coupled to one terminal of the photodiode and receives the photocurrent. The remaining terminal of the photodiode is connected to a fixed potential, for example. In addition, the inverting input is connected to the output of the opamp via a feedback resistor, and the non-inverting input may be coupled to a fixed potential, such as ground. In this configuration, the output voltage of the opamp has a relatively fast response time and is equal to the input current multiplied by the resistance of the feedback resistor.
In order to insure proper operation of an optical receiver, an appropriate bias is applied across the photodiode. This bias is based on a difference between the voltage of the inverting input of the opamp, which, as noted above, is connected to a first photodiode terminal, and the fixed potential supplied to the second photodiode terminal. The voltage of the inverting opamp input, however, can vary with temperature and may vary from one chip to the next due to process variations. Without direct measurement of the inverting opamp input voltage, therefore, the appropriate photodiode bias may be difficult to determine.
Accordingly, a convenient and accurate mechanism for determining the inverting opamp input voltage, and thus, the photodiode bias, is desired.